/*
 * Copyright (c) 2018 - Manifold Systems LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package manifold.util.concurrent;

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

import java.io.IOException;
import java.io.Serializable;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.locks.ReentrantLock;

/**
 * A hash table with <em>weak keys</em>, full concurrency of retrievals, and
 * adjustable expected concurrency for updates. Similar to
 * {@link java.util.WeakHashMap}, entries of this table are periodically
 * removed once their corresponding keys are no longer referenced outside of
 * this table. In other words, this table will not prevent a key from being
 * discarded by the garbage collector. Once a key has been discarded by the
 * collector, the corresponding entry is no longer visible to this table;
 * however, the entry may occupy space until a future table operation decides to
 * reclaim it. For this reason, summary functions such as <tt>size</tt> and
 * <tt>isEmpty</tt> might return a value greater than the observed number of
 * entries. In order to support a high level of concurrency, stale entries are
 * only reclaimed during blocking (usually mutating) operations.
 * <p>
 * While keys in this table are only held using a weak reference, values are
 * held using a normal strong reference. This provides the guarantee that a
 * value will always have at least the same life-span as it's key. For this
 * reason, care should be taken to ensure that a value never refers, either
 * directly or indirectly, to its key, thereby preventing reclamation. If weak
 * values are desired, one can simply use a {@link WeakReference} for the value
 * type.
 * <p>
 * Just like {@link java.util.concurrent.ConcurrentHashMap}, this class obeys the same
 * functional specification as {@link Hashtable}, and includes
 * versions of methods corresponding to each method of <tt>Hashtable</tt>.
 * However, even though all operations are thread-safe, retrieval operations do
 * <em>not</em> entail locking, and there is <em>not</em> any support for
 * locking the entire table in a way that prevents all access. This class is
 * fully interoperable with <tt>Hashtable</tt> in programs that rely on its
 * thread safety but not on its synchronization details.
 * <p>
 * <p>
 * Retrieval operations (including <tt>get</tt>) generally do not block, so
 * may overlap with update operations (including <tt>put</tt> and
 * <tt>remove</tt>). Retrievals reflect the results of the most recently
 * <em>completed</em> update operations holding upon their onset. For
 * aggregate operations such as <tt>putAll</tt> and <tt>clear</tt>,
 * concurrent retrievals may reflect insertion or removal of only some entries.
 * Similarly, Iterators and Enumerations return elements reflecting the state of
 * the hash table at some point at or since the creation of the
 * iterator/enumeration. They do <em>not</em> throw
 * {@link ConcurrentModificationException}. However, iterators are designed to
 * be used by only one thread at a time.
 * <p>
 * <p>
 * The allowed concurrency among update operations is guided by the optional
 * <tt>concurrencyLevel</tt> constructor argument (default <tt>16</tt>),
 * which is used as a hint for internal sizing. The table is internally
 * partitioned to try to permit the indicated number of concurrent updates
 * without contention. Because placement in hash tables is essentially random,
 * the actual concurrency will vary. Ideally, you should choose a value to
 * accommodate as many threads as will ever concurrently modify the table. Using
 * a significantly higher value than you need can waste space and time, and a
 * significantly lower value can lead to thread contention. But overestimates
 * and underestimates within an order of magnitude do not usually have much
 * noticeable impact. A value of one is appropriate when it is known that only
 * one thread will modify and all others will only read. Also, resizing this or
 * any other kind of hash table is a relatively slow operation, so, when
 * possible, it is a good idea to provide estimates of expected table sizes in
 * constructors.
 * <p>
 * <p>
 * This class and its views and iterators implement all of the <em>optional</em>
 * methods of the {@link Map} and {@link Iterator} interfaces.
 * <p>
 * <p>
 * Like {@link Hashtable} but unlike {@link HashMap}, this class does
 * <em>not</em> allow <tt>null</tt> to be used as a key or value.
 * <p>
 * <p>
 * This class is a member of the <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author Doug Lea
 * @author Jason T. Greene
 */
public class ConcurrentWeakHashMap<K, V> extends AbstractMap<K, V>
  implements java.util.concurrent.ConcurrentMap<K, V>, Serializable
{
  private static final long serialVersionUID = 7249069246763182397L;

    /*
     * The basic strategy is to subdivide the table among Segments,
     * each of which itself is a concurrently readable hash table.
     */

    /* ---------------- Constants -------------- */

  /**
   * The default demo capacity for this table,
   * used when not otherwise specified in a constructor.
   */
  static final int DEFAULT_INITIAL_CAPACITY = 16;

  /**
   * The default load factor for this table, used when not
   * otherwise specified in a constructor.
   */
  static final float DEFAULT_LOAD_FACTOR = 0.75f;

  /**
   * The default concurrency level for this table, used when not
   * otherwise specified in a constructor.
   */
  static final int DEFAULT_CONCURRENCY_LEVEL = 16;

  /**
   * The maximum capacity, used if a higher value is implicitly
   * specified by either of the constructors with arguments.  MUST
   * be a power of two <= 1<<30 to ensure that entries are indexable
   * using ints.
   */
  static final int MAXIMUM_CAPACITY = 1 << 30;

  /**
   * The maximum number of segments to allow; used to bound
   * constructor arguments.
   */
  static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

  /**
   * Number of unsynchronized retries in size and containsValue
   * methods before resorting to locking. This is used to avoid
   * unbounded retries if tables undergo continuous modification
   * which would make it impossible to obtain an accurate result.
   */
  static final int RETRIES_BEFORE_LOCK = 2;

    /* ---------------- Fields -------------- */

  /**
   * Mask value for indexing into segments. The upper bits of a
   * key's hash code are used to choose the segment.
   */
  final int segmentMask;

  /**
   * Shift value for indexing within segments.
   */
  final int segmentShift;

  /**
   * The segments, each of which is a specialized hash table
   */
  final Segment<K, V>[] segments;

  transient Set<K> keySet;
  transient Set<Entry<K, V>> entrySet;
  transient Collection<V> values;

    /* ---------------- Small Utilities -------------- */

  /**
   * Applies a supplemental hash function to a given hashCode, which
   * defends against poor quality hash functions.  This is critical
   * because ConcurrentWeakHashMap uses power-of-two length hash tables,
   * that otherwise encounter collisions for hashCodes that do not
   * differ in lower or upper bits.
   */
  private static int hash( int h )
  {
    // Spread bits to regularize both segment and index locations,
    // using variant of single-word Wang/Jenkins hash.
    h += (h << 15) ^ 0xffffcd7d;
    h ^= (h >>> 10);
    h += (h << 3);
    h ^= (h >>> 6);
    h += (h << 2) + (h << 14);
    return h ^ (h >>> 16);
  }

  /**
   * Returns the segment that should be used for key with given hash
   *
   * @param hash the hash code for the key
   *
   * @return the segment
   */
  final Segment<K, V> segmentFor( int hash )
  {
    return segments[(hash >>> segmentShift) & segmentMask];
  }

    /* ---------------- Inner Classes -------------- */

  /**
   * A weak-key reference which stores the key hash needed for reclamation.
   */
  static final class WeakKeyReference<K> extends WeakReference<K>
  {
    final int hash;

    WeakKeyReference( K key, int hash, ReferenceQueue<K> refQueue )
    {
      super( key, refQueue );
      this.hash = hash;
    }
  }

  /**
   * ConcurrentWeakHashMap list entry. Note that this is never exported
   * out as a user-visible Map.Entry.
   * <p>
   * Because the value field is volatile, not final, it is legal wrt
   * the Java Memory Model for an unsynchronized reader to see null
   * instead of demo value when read via a data race.  Although a
   * reordering leading to this is not likely to ever actually
   * occur, the Segment.readValueUnderLock method is used as a
   * backup in case a null (pre-initialized) value is ever seen in
   * an unsynchronized access method.
   */
  static final class HashEntry<K, V>
  {
    final WeakReference<K> keyRef;
    final int hash;
    volatile V value;
    final HashEntry<K, V> next;

    HashEntry( K key, int hash, HashEntry<K, V> next, V value, ReferenceQueue<K> refQueue )
    {
      this.keyRef = new WeakKeyReference<K>( key, hash, refQueue );
      this.hash = hash;
      this.next = next;
      this.value = value;
    }

    @SuppressWarnings("unchecked")
    static final <K, V> HashEntry<K, V>[] newArray( int i )
    {
      return new HashEntry[i];
    }
  }

  /**
   * Segments are specialized versions of hash tables.  This
   * subclasses from ReentrantLock opportunistically, just to
   * simplify some locking and avoid separate construction.
   */
  static final class Segment<K, V> extends ReentrantLock implements Serializable
  {
        /*
         * Segments maintain a table of entry lists that are ALWAYS
         * kept in a consistent state, so can be read without locking.
         * Next fields of nodes are immutable (final).  All list
         * additions are performed at the front of each bin. This
         * makes it easy to check changes, and also fast to traverse.
         * When nodes would otherwise be changed, new nodes are
         * created to replace them. This works well for hash tables
         * since the bin lists tend to be short. (The average length
         * is less than two for the default load factor threshold.)
         *
         * Read operations can thus proceed without locking, but rely
         * on selected uses of volatiles to ensure that completed
         * write operations performed by other threads are
         * noticed. For most purposes, the "count" field, tracking the
         * number of elements, serves as that volatile variable
         * ensuring visibility.  This is convenient because this field
         * needs to be read in many read operations anyway:
         *
         *   - All (unsynchronized) read operations must first read the
         *     "count" field, and should not look at table entries if
         *     it is 0.
         *
         *   - All (synchronized) write operations should write to
         *     the "count" field after structurally changing any bin.
         *     The operations must not take any action that could even
         *     momentarily cause a concurrent read operation to see
         *     inconsistent data. This is made easier by the nature of
         *     the read operations in Map. For example, no operation
         *     can reveal that the table has grown but the threshold
         *     has not yet been updated, so there are no atomicity
         *     requirements for this with respect to reads.
         *
         * As a guide, all critical volatile reads and writes to the
         * count field are marked in code comments.
         */

    private static final long serialVersionUID = 2249069246763182397L;

    /**
     * The number of elements in this segment's region.
     */
    transient volatile int count;

    /**
     * Number of updates that alter the size of the table. This is
     * used during bulk-read methods to make sure they see a
     * consistent snapshot: If modCounts change during a traversal
     * of segments computing size or checking containsValue, then
     * we might have an inconsistent view of state so (usually)
     * must retry.
     */
    transient int modCount;

    /**
     * The table is rehashed when its size exceeds this threshold.
     * (The value of this field is always <tt>(int)(capacity *
     * loadFactor)</tt>.)
     */
    transient int threshold;

    /**
     * The per-segment table.
     */
    transient volatile HashEntry<K, V>[] table;

    /**
     * The load factor for the hash table.  Even though this value
     * is same for all segments, it is replicated to avoid needing
     * links to outer object.
     *
     * @serial
     */
    final float loadFactor;

    /**
     * The collected weak-key reference queue for this segment.
     * This should be (re)initialized whenever table is assigned,
     */
    transient volatile ReferenceQueue<K> refQueue;

    Segment( int initialCapacity, float lf )
    {
      loadFactor = lf;
      setTable( HashEntry.<K, V>newArray( initialCapacity ) );
    }

    @SuppressWarnings("unchecked")
    static final <K, V> Segment<K, V>[] newArray( int i )
    {
      return new Segment[i];
    }

    /**
     * Sets table to new HashEntry array.
     * Call only while holding lock or in constructor.
     */
    void setTable( HashEntry<K, V>[] newTable )
    {
      threshold = (int)(newTable.length * loadFactor);
      table = newTable;
      refQueue = new ReferenceQueue<K>();
    }

    /**
     * Returns properly casted first entry of bin for given hash.
     */
    HashEntry<K, V> getFirst( int hash )
    {
      HashEntry<K, V>[] tab = table;
      return tab[hash & (tab.length - 1)];
    }

    /**
     * Reads value field of an entry under lock. Called if value
     * field ever appears to be null. This is possible only if a
     * compiler happens to reorder a HashEntry initialization with
     * its table assignment, which is legal under memory model
     * but is not known to ever occur.
     */
    V readValueUnderLock( HashEntry<K, V> e )
    {
      lock();
      try
      {
        removeStale();
        return e.value;
      }
      finally
      {
        unlock();
      }
    }

        /* Specialized implementations of map methods */

    V get( Object key, int hash )
    {
      if( count != 0 )
      { // read-volatile
        HashEntry<K, V> e = getFirst( hash );
        while( e != null )
        {
          if( e.hash == hash && key.equals( e.keyRef.get() ) )
          {
            V v = e.value;
            if( v != null )
            {
              return v;
            }
            return readValueUnderLock( e ); // recheck
          }
          e = e.next;
        }
      }
      return null;
    }

    boolean containsKey( Object key, int hash )
    {
      if( count != 0 )
      { // read-volatile
        HashEntry<K, V> e = getFirst( hash );
        while( e != null )
        {
          if( e.hash == hash && key.equals( e.keyRef.get() ) )
          {
            return true;
          }
          e = e.next;
        }
      }
      return false;
    }

    boolean containsValue( Object value )
    {
      if( count != 0 )
      { // read-volatile
        HashEntry<K, V>[] tab = table;
        int len = tab.length;
        for( int i = 0; i < len; i++ )
        {
          for( HashEntry<K, V> e = tab[i]; e != null; e = e.next )
          {
            V v = e.value;
            if( v == null ) // recheck
            {
              v = readValueUnderLock( e );
            }
            if( value.equals( v ) )
            {
              return true;
            }
          }
        }
      }
      return false;
    }

    boolean replace( K key, int hash, V oldValue, V newValue )
    {
      lock();
      try
      {
        removeStale();
        HashEntry<K, V> e = getFirst( hash );
        while( e != null && (e.hash != hash || !key.equals( e.keyRef.get() )) )
        {
          e = e.next;
        }

        boolean replaced = false;
        if( e != null && oldValue.equals( e.value ) )
        {
          replaced = true;
          e.value = newValue;
        }
        return replaced;
      }
      finally
      {
        unlock();
      }
    }

    V replace( K key, int hash, V newValue )
    {
      lock();
      try
      {
        removeStale();
        HashEntry<K, V> e = getFirst( hash );
        while( e != null && (e.hash != hash || !key.equals( e.keyRef.get() )) )
        {
          e = e.next;
        }

        V oldValue = null;
        if( e != null )
        {
          oldValue = e.value;
          e.value = newValue;
        }
        return oldValue;
      }
      finally
      {
        unlock();
      }
    }

    V put( K key, int hash, V value, boolean onlyIfAbsent )
    {
      lock();
      try
      {
        removeStale();
        int c = count;
        if( c++ > threshold )
        {// ensure capacity
          int reduced = rehash();
          if( reduced > 0 ) // adjust from possible weak cleanups
          {
            count = (c -= reduced) - 1; // write-volatile
          }
        }

        HashEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        HashEntry<K, V> first = tab[index];
        HashEntry<K, V> e = first;
        while( e != null && (e.hash != hash || !key.equals( e.keyRef.get() )) )
        {
          e = e.next;
        }

        V oldValue;
        if( e != null )
        {
          oldValue = e.value;
          if( !onlyIfAbsent )
          {
            e.value = value;
          }
        }
        else
        {
          oldValue = null;
          ++modCount;
          tab[index] = new HashEntry<K, V>( key, hash, first, value, refQueue );
          count = c; // write-volatile
        }
        return oldValue;
      }
      finally
      {
        unlock();
      }
    }

    int rehash()
    {
      HashEntry<K, V>[] oldTable = table;
      int oldCapacity = oldTable.length;
      if( oldCapacity >= MAXIMUM_CAPACITY )
      {
        return 0;
      }

            /*
             * Reclassify nodes in each list to new Map.  Because we are
             * using power-of-two expansion, the elements from each bin
             * must either stay at same index, or move with a power of two
             * offset. We eliminate unnecessary node creation by catching
             * cases where old nodes can be reused because their next
             * fields won't change. Statistically, at the default
             * threshold, only about one-sixth of them need cloning when
             * a table doubles. The nodes they replace will be garbage
             * collectable as soon as they are no longer referenced by any
             * reader thread that may be in the midst of traversing table
             * right now.
             */

      HashEntry<K, V>[] newTable = HashEntry.newArray( oldCapacity << 1 );
      threshold = (int)(newTable.length * loadFactor);
      int sizeMask = newTable.length - 1;
      int reduce = 0;
      for( int i = 0; i < oldCapacity; i++ )
      {
        // We need to guarantee that any existing reads of old Map can
        //  proceed. So we cannot yet null out each bin.
        HashEntry<K, V> e = oldTable[i];

        if( e != null )
        {
          HashEntry<K, V> next = e.next;
          int idx = e.hash & sizeMask;

          //  Single node on list
          if( next == null )
          {
            newTable[idx] = e;
          }

          else
          {
            // Reuse trailing consecutive sequence at same slot
            HashEntry<K, V> lastRun = e;
            int lastIdx = idx;
            for( HashEntry<K, V> last = next; last != null; last = last.next )
            {
              int k = last.hash & sizeMask;
              if( k != lastIdx )
              {
                lastIdx = k;
                lastRun = last;
              }
            }
            newTable[lastIdx] = lastRun;
            // Clone all remaining nodes
            for( HashEntry<K, V> p = e; p != lastRun; p = p.next )
            {
              // Skip GC'd weak refs
              K key = p.keyRef.get();
              if( key == null )
              {
                reduce++;
                continue;
              }
              int k = p.hash & sizeMask;
              HashEntry<K, V> n = newTable[k];
              newTable[k] = new HashEntry<K, V>( key, p.hash, n, p.value, refQueue );
            }
          }
        }
      }
      table = newTable;
      return reduce;
    }

    /**
     * Remove; match on key only if value null, else match both.
     */
    V remove( Object key, int hash, Object value, boolean weakRemove )
    {
      lock();
      try
      {
        if( !weakRemove )
        {
          removeStale();
        }
        int c = count - 1;
        HashEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        HashEntry<K, V> first = tab[index];
        HashEntry<K, V> e = first;
        // a weak remove operation compares the WeakReference instance
        while( e != null && (!weakRemove || key != e.keyRef)
               && (e.hash != hash || !key.equals( e.keyRef.get() )) )
        {
          e = e.next;
        }

        V oldValue = null;
        if( e != null )
        {
          V v = e.value;
          if( value == null || value.equals( v ) )
          {
            oldValue = v;
            // All entries following removed node can stay
            // in list, but all preceding ones need to be
            // cloned.
            ++modCount;
            HashEntry<K, V> newFirst = e.next;
            for( HashEntry<K, V> p = first; p != e; p = p.next )
            {
              K pKey = p.keyRef.get();
              if( pKey == null )
              { // Skip GC'd keys
                c--;
                continue;
              }

              newFirst = new HashEntry<K, V>( pKey, p.hash, newFirst, p.value, refQueue );
            }
            tab[index] = newFirst;
            count = c; // write-volatile
          }
        }
        return oldValue;
      }
      finally
      {
        unlock();
      }
    }

    @SuppressWarnings("unchecked")
    void removeStale()
    {
      WeakKeyReference<K> ref;
      while( (ref = (WeakKeyReference<K>)refQueue.poll()) != null )
      {
        remove( ref, ref.hash, null, true );
      }
    }

    void clear()
    {
      if( count != 0 )
      {
        lock();
        try
        {
          HashEntry<K, V>[] tab = table;
          for( int i = 0; i < tab.length; i++ )
          {
            tab[i] = null;
          }
          ++modCount;
          // replace the reference queue to avoid unnecessary stale cleanups
          refQueue = new ReferenceQueue<K>();
          count = 0; // write-volatile
        }
        finally
        {
          unlock();
        }
      }
    }
  }

    /* ---------------- Public operations -------------- */

  /**
   * Creates a new, empty map with the specified demo
   * capacity, load factor and concurrency level.
   *
   * @param initialCapacity  the demo capacity. The implementation
   *                         performs internal sizing to accommodate this many elements.
   * @param loadFactor       the load factor threshold, used to control resizing.
   *                         Resizing may be performed when the average number of elements per
   *                         bin exceeds this threshold.
   * @param concurrencyLevel the estimated number of concurrently
   *                         updating threads. The implementation performs internal sizing
   *                         to try to accommodate this many threads.
   *
   * @throws IllegalArgumentException if the demo capacity is
   *                                  negative or the load factor or concurrencyLevel are
   *                                  nonpositive.
   */
  public ConcurrentWeakHashMap( int initialCapacity, float loadFactor, int concurrencyLevel )
  {
    if( !(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0 )
    {
      throw new IllegalArgumentException();
    }

    if( concurrencyLevel > MAX_SEGMENTS )
    {
      concurrencyLevel = MAX_SEGMENTS;
    }

    // Find power-of-two sizes best matching arguments
    int sshift = 0;
    int ssize = 1;
    while( ssize < concurrencyLevel )
    {
      ++sshift;
      ssize <<= 1;
    }
    segmentShift = 32 - sshift;
    segmentMask = ssize - 1;
    this.segments = Segment.newArray( ssize );

    if( initialCapacity > MAXIMUM_CAPACITY )
    {
      initialCapacity = MAXIMUM_CAPACITY;
    }
    int c = initialCapacity / ssize;
    if( c * ssize < initialCapacity )
    {
      ++c;
    }
    int cap = 1;
    while( cap < c )
    {
      cap <<= 1;
    }

    for( int i = 0; i < this.segments.length; ++i )
    {
      this.segments[i] = new Segment<K, V>( cap, loadFactor );
    }
  }

  /**
   * Creates a new, empty map with the specified demo capacity
   * and load factor and with the default concurrencyLevel (16).
   *
   * @param initialCapacity The implementation performs internal
   *                        sizing to accommodate this many elements.
   * @param loadFactor      the load factor threshold, used to control resizing.
   *                        Resizing may be performed when the average number of elements per
   *                        bin exceeds this threshold.
   *
   * @throws IllegalArgumentException if the demo capacity of
   *                                  elements is negative or the load factor is nonpositive
   * @since 1.6
   */
  public ConcurrentWeakHashMap( int initialCapacity, float loadFactor )
  {
    this( initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL );
  }

  /**
   * Creates a new, empty map with the specified demo capacity,
   * and with default load factor (0.75) and concurrencyLevel (16).
   *
   * @param initialCapacity the demo capacity. The implementation
   *                        performs internal sizing to accommodate this many elements.
   *
   * @throws IllegalArgumentException if the demo capacity of
   *                                  elements is negative.
   */
  public ConcurrentWeakHashMap( int initialCapacity )
  {
    this( initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL );
  }

  /**
   * Creates a new, empty map with a default demo capacity (16),
   * load factor (0.75) and concurrencyLevel (16).
   */
  public ConcurrentWeakHashMap()
  {
    this( DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL );
  }

  /**
   * Creates a new map with the same mappings as the given map.
   * The map is created with a capacity of 1.5 times the number
   * of mappings in the given map or 16 (whichever is greater),
   * and a default load factor (0.75) and concurrencyLevel (16).
   *
   * @param m the map
   */
  public ConcurrentWeakHashMap( Map<? extends K, ? extends V> m )
  {
    this( Math.max( (int)(m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY ), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL );
    putAll( m );
  }

  /**
   * Returns <tt>true</tt> if this map contains no key-value mappings.
   *
   * @return <tt>true</tt> if this map contains no key-value mappings
   */
  public boolean isEmpty()
  {
    final Segment<K, V>[] segments = this.segments;
        /*
         * We keep track of per-segment modCounts to avoid ABA
         * problems in which an element in one segment was added and
         * in another removed during traversal, in which case the
         * table was never actually empty at any point. Note the
         * similar use of modCounts in the size() and containsValue()
         * methods, which are the only other methods also susceptible
         * to ABA problems.
         */
    int[] mc = new int[segments.length];
    int mcsum = 0;
    for( int i = 0; i < segments.length; ++i )
    {
      if( segments[i].count != 0 )
      {
        return false;
      }
      else
      {
        mcsum += mc[i] = segments[i].modCount;
      }
    }
    // If mcsum happens to be zero, then we know we got a snapshot
    // before any modifications at all were made.  This is
    // probably common enough to bother tracking.
    if( mcsum != 0 )
    {
      for( int i = 0; i < segments.length; ++i )
      {
        if( segments[i].count != 0 || mc[i] != segments[i].modCount )
        {
          return false;
        }
      }
    }
    return true;
  }

  /**
   * Returns the number of key-value mappings in this map.  If the
   * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
   * <tt>Integer.MAX_VALUE</tt>.
   *
   * @return the number of key-value mappings in this map
   */
  public int size()
  {
    final Segment<K, V>[] segments = this.segments;
    long sum = 0;
    long check = 0;
    int[] mc = new int[segments.length];
    // Try a few times to get accurate count. On failure due to
    // continuous async changes in table, resort to locking.
    for( int k = 0; k < RETRIES_BEFORE_LOCK; ++k )
    {
      check = 0;
      sum = 0;
      int mcsum = 0;
      for( int i = 0; i < segments.length; ++i )
      {
        sum += segments[i].count;
        mcsum += mc[i] = segments[i].modCount;
      }
      if( mcsum != 0 )
      {
        for( int i = 0; i < segments.length; ++i )
        {
          check += segments[i].count;
          if( mc[i] != segments[i].modCount )
          {
            check = -1; // force retry
            break;
          }
        }
      }
      if( check == sum )
      {
        break;
      }
    }
    if( check != sum )
    { // Resort to locking all segments
      sum = 0;
      for( int i = 0; i < segments.length; ++i )
      {
        segments[i].lock();
      }
      for( int i = 0; i < segments.length; ++i )
      {
        sum += segments[i].count;
      }
      for( int i = 0; i < segments.length; ++i )
      {
        segments[i].unlock();
      }
    }
    if( sum > Integer.MAX_VALUE )
    {
      return Integer.MAX_VALUE;
    }
    else
    {
      return (int)sum;
    }
  }

  /**
   * Returns the value to which the specified key is mapped,
   * or {@code null} if this map contains no mapping for the key.
   * <p>
   * <p>More formally, if this map contains a mapping from a key
   * {@code k} to a value {@code v} such that {@code key.equals(k)},
   * then this method returns {@code v}; otherwise it returns
   * {@code null}.  (There can be at most one such mapping.)
   *
   * @throws NullPointerException if the specified key is null
   */
  public V get( Object key )
  {
    int hash = hash( key.hashCode() );
    return segmentFor( hash ).get( key, hash );
  }

  /**
   * Tests if the specified object is a key in this table.
   *
   * @param key possible key
   *
   * @return <tt>true</tt> if and only if the specified object
   * is a key in this table, as determined by the
   * <tt>equals</tt> method; <tt>false</tt> otherwise.
   *
   * @throws NullPointerException if the specified key is null
   */
  public boolean containsKey( Object key )
  {
    int hash = hash( key.hashCode() );
    return segmentFor( hash ).containsKey( key, hash );
  }

  /**
   * Returns <tt>true</tt> if this map maps one or more keys to the
   * specified value. Note: This method requires a full internal
   * traversal of the hash table, and so is much slower than
   * method <tt>containsKey</tt>.
   *
   * @param value value whose presence in this map is to be tested
   *
   * @return <tt>true</tt> if this map maps one or more keys to the
   * specified value
   *
   * @throws NullPointerException if the specified value is null
   */
  public boolean containsValue( Object value )
  {
    if( value == null )
    {
      throw new NullPointerException();
    }

    // See explanation of modCount use above

    final Segment<K, V>[] segments = this.segments;
    int[] mc = new int[segments.length];

    // Try a few times without locking
    for( int k = 0; k < RETRIES_BEFORE_LOCK; ++k )
    {
      int sum = 0;
      int mcsum = 0;
      for( int i = 0; i < segments.length; ++i )
      {
        int c = segments[i].count;
        mcsum += mc[i] = segments[i].modCount;
        if( segments[i].containsValue( value ) )
        {
          return true;
        }
      }
      boolean cleanSweep = true;
      if( mcsum != 0 )
      {
        for( int i = 0; i < segments.length; ++i )
        {
          int c = segments[i].count;
          if( mc[i] != segments[i].modCount )
          {
            cleanSweep = false;
            break;
          }
        }
      }
      if( cleanSweep )
      {
        return false;
      }
    }
    // Resort to locking all segments
    for( int i = 0; i < segments.length; ++i )
    {
      segments[i].lock();
    }
    boolean found = false;
    try
    {
      for( int i = 0; i < segments.length; ++i )
      {
        if( segments[i].containsValue( value ) )
        {
          found = true;
          break;
        }
      }
    }
    finally
    {
      for( int i = 0; i < segments.length; ++i )
      {
        segments[i].unlock();
      }
    }
    return found;
  }

  /**
   * Legacy method testing if some key maps into the specified value
   * in this table.  This method is identical in functionality to
   * {@link #containsValue}, and exists solely to ensure
   * full compatibility with class {@link Hashtable},
   * which supported this method prior to introduction of the
   * Java Collections framework.
   *
   * @param value a value to search for
   *
   * @return <tt>true</tt> if and only if some key maps to the
   * <tt>value</tt> argument in this table as
   * determined by the <tt>equals</tt> method;
   * <tt>false</tt> otherwise
   *
   * @throws NullPointerException if the specified value is null
   */
  public boolean contains( Object value )
  {
    return containsValue( value );
  }

  /**
   * Maps the specified key to the specified value in this table.
   * Neither the key nor the value can be null.
   * <p>
   * <p> The value can be retrieved by calling the <tt>get</tt> method
   * with a key that is equal to the original key.
   *
   * @param key   key with which the specified value is to be associated
   * @param value value to be associated with the specified key
   *
   * @return the previous value associated with <tt>key</tt>, or
   * <tt>null</tt> if there was no mapping for <tt>key</tt>
   *
   * @throws NullPointerException if the specified key or value is null
   */
  public V put( K key, V value )
  {
    if( value == null )
    {
      throw new NullPointerException();
    }
    int hash = hash( key.hashCode() );
    return segmentFor( hash ).put( key, hash, value, false );
  }

  /**
   * {@inheritDoc}
   *
   * @return the previous value associated with the specified key,
   * or <tt>null</tt> if there was no mapping for the key
   *
   * @throws NullPointerException if the specified key or value is null
   */
  public V putIfAbsent( K key, V value )
  {
    if( value == null )
    {
      throw new NullPointerException();
    }
    int hash = hash( key.hashCode() );
    return segmentFor( hash ).put( key, hash, value, true );
  }

  /**
   * Copies all of the mappings from the specified map to this one.
   * These mappings replace any mappings that this map had for any of the
   * keys currently in the specified map.
   *
   * @param m mappings to be stored in this map
   */
  public void putAll( Map<? extends K, ? extends V> m )
  {
    for( Entry<? extends K, ? extends V> e : m.entrySet() )
    {
      put( e.getKey(), e.getValue() );
    }
  }

  /**
   * Removes the key (and its corresponding value) from this map.
   * This method does nothing if the key is not in the map.
   *
   * @param key the key that needs to be removed
   *
   * @return the previous value associated with <tt>key</tt>, or
   * <tt>null</tt> if there was no mapping for <tt>key</tt>
   *
   * @throws NullPointerException if the specified key is null
   */
  public V remove( Object key )
  {
    int hash = hash( key.hashCode() );
    return segmentFor( hash ).remove( key, hash, null, false );
  }

  /**
   * {@inheritDoc}
   *
   * @throws NullPointerException if the specified key is null
   */
  public boolean remove( Object key, Object value )
  {
    int hash = hash( key.hashCode() );
    if( value == null )
    {
      return false;
    }
    return segmentFor( hash ).remove( key, hash, value, false ) != null;
  }

  /**
   * {@inheritDoc}
   *
   * @throws NullPointerException if any of the arguments are null
   */
  public boolean replace( K key, V oldValue, V newValue )
  {
    if( oldValue == null || newValue == null )
    {
      throw new NullPointerException();
    }
    int hash = hash( key.hashCode() );
    return segmentFor( hash ).replace( key, hash, oldValue, newValue );
  }

  /**
   * {@inheritDoc}
   *
   * @return the previous value associated with the specified key,
   * or <tt>null</tt> if there was no mapping for the key
   *
   * @throws NullPointerException if the specified key or value is null
   */
  public V replace( K key, V value )
  {
    if( value == null )
    {
      throw new NullPointerException();
    }
    int hash = hash( key.hashCode() );
    return segmentFor( hash ).replace( key, hash, value );
  }

  /**
   * Removes all of the mappings from this map.
   */
  public void clear()
  {
    for( int i = 0; i < segments.length; ++i )
    {
      segments[i].clear();
    }
  }

  /**
   * Returns a {@link Set} view of the keys contained in this map.
   * The set is backed by the map, so changes to the map are
   * reflected in the set, and vice-versa.  The set supports element
   * removal, which removes the corresponding mapping from this map,
   * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
   * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
   * operations.  It does not support the <tt>add</tt> or
   * <tt>addAll</tt> operations.
   * <p>
   * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
   * that will never throw {@link ConcurrentModificationException},
   * and guarantees to traverse elements as they existed upon
   * construction of the iterator, and may (but is not guaranteed to)
   * reflect any modifications subsequent to construction.
   */
  public Set<K> keySet()
  {
    Set<K> ks = keySet;
    return (ks != null) ? ks : (keySet = new KeySet());
  }

  /**
   * Returns a {@link Collection} view of the values contained in this map.
   * The collection is backed by the map, so changes to the map are
   * reflected in the collection, and vice-versa.  The collection
   * supports element removal, which removes the corresponding
   * mapping from this map, via the <tt>Iterator.remove</tt>,
   * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
   * <tt>retainAll</tt>, and <tt>clear</tt> operations.  It does not
   * support the <tt>add</tt> or <tt>addAll</tt> operations.
   * <p>
   * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
   * that will never throw {@link ConcurrentModificationException},
   * and guarantees to traverse elements as they existed upon
   * construction of the iterator, and may (but is not guaranteed to)
   * reflect any modifications subsequent to construction.
   */
  public Collection<V> values()
  {
    Collection<V> vs = values;
    return (vs != null) ? vs : (values = new Values());
  }

  /**
   * Returns a {@link Set} view of the mappings contained in this map.
   * The set is backed by the map, so changes to the map are
   * reflected in the set, and vice-versa.  The set supports element
   * removal, which removes the corresponding mapping from the map,
   * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
   * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
   * operations.  It does not support the <tt>add</tt> or
   * <tt>addAll</tt> operations.
   * <p>
   * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
   * that will never throw {@link ConcurrentModificationException},
   * and guarantees to traverse elements as they existed upon
   * construction of the iterator, and may (but is not guaranteed to)
   * reflect any modifications subsequent to construction.
   */
  public Set<Entry<K, V>> entrySet()
  {
    Set<Entry<K, V>> es = entrySet;
    return (es != null) ? es : (entrySet = new EntrySet());
  }

  /**
   * Returns an enumeration of the keys in this table.
   *
   * @return an enumeration of the keys in this table
   *
   * @see #keySet()
   */
  public Enumeration<K> keys()
  {
    return new KeyIterator();
  }

  /**
   * Returns an enumeration of the values in this table.
   *
   * @return an enumeration of the values in this table
   *
   * @see #values()
   */
  public Enumeration<V> elements()
  {
    return new ValueIterator();
  }

    /* ---------------- Iterator Support -------------- */

  abstract class HashIterator
  {
    int nextSegmentIndex;
    int nextTableIndex;
    HashEntry<K, V>[] currentTable;
    HashEntry<K, V> nextEntry;
    HashEntry<K, V> lastReturned;
    K currentKey; // Strong reference to weak key (prevents gc)

    HashIterator()
    {
      nextSegmentIndex = segments.length - 1;
      nextTableIndex = -1;
      advance();
    }

    public boolean hasMoreElements()
    {
      return hasNext();
    }

    final void advance()
    {
      if( nextEntry != null && (nextEntry = nextEntry.next) != null )
      {
        return;
      }

      while( nextTableIndex >= 0 )
      {
        if( (nextEntry = currentTable[nextTableIndex--]) != null )
        {
          return;
        }
      }

      while( nextSegmentIndex >= 0 )
      {
        Segment<K, V> seg = segments[nextSegmentIndex--];
        if( seg.count != 0 )
        {
          currentTable = seg.table;
          for( int j = currentTable.length - 1; j >= 0; --j )
          {
            if( (nextEntry = currentTable[j]) != null )
            {
              nextTableIndex = j - 1;
              return;
            }
          }
        }
      }
    }

    public boolean hasNext()
    {
      while( nextEntry != null )
      {
        if( nextEntry.keyRef.get() != null )
        {
          return true;
        }
        advance();
      }

      return false;
    }

    HashEntry<K, V> nextEntry()
    {
      do
      {
        if( nextEntry == null )
        {
          throw new NoSuchElementException();
        }

        lastReturned = nextEntry;
        currentKey = lastReturned.keyRef.get();
        advance();
      } while( currentKey == null ); // Skip GC'd keys

      return lastReturned;
    }

    public void remove()
    {
      if( lastReturned == null )
      {
        throw new IllegalStateException();
      }
      ConcurrentWeakHashMap.this.remove( currentKey );
      lastReturned = null;
    }
  }

  final class KeyIterator extends HashIterator implements Iterator<K>, Enumeration<K>
  {
    public K next()
    {
      return super.nextEntry().keyRef.get();
    }

    public K nextElement()
    {
      return super.nextEntry().keyRef.get();
    }
  }

  final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V>
  {
    public V next()
    {
      return super.nextEntry().value;
    }

    public V nextElement()
    {
      return super.nextEntry().value;
    }
  }

  /*
   * This class is needed for JDK5 compatibility.
   */
  static class SimpleEntry<K, V> implements Entry<K, V>, Serializable
  {
    private static final long serialVersionUID = -8499721149061103585L;

    private final K key;
    private V value;

    public SimpleEntry( K key, V value )
    {
      this.key = key;
      this.value = value;
    }

    public SimpleEntry( Entry<? extends K, ? extends V> entry )
    {
      this.key = entry.getKey();
      this.value = entry.getValue();
    }

    public K getKey()
    {
      return key;
    }

    public V getValue()
    {
      return value;
    }

    public V setValue( V value )
    {
      V oldValue = this.value;
      this.value = value;
      return oldValue;
    }

    public boolean equals( Object o )
    {
      if( !(o instanceof Map.Entry) )
      {
        return false;
      }
      @SuppressWarnings("unchecked")
      Entry e = (Entry)o;
      return eq( key, e.getKey() ) && eq( value, e.getValue() );
    }

    public int hashCode()
    {
      return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
    }

    public String toString()
    {
      return key + "=" + value;
    }

    private static boolean eq( Object o1, Object o2 )
    {
      return o1 == null ? o2 == null : o1.equals( o2 );
    }
  }

  /**
   * Custom Entry class used by EntryIterator.next(), that relays setValue
   * changes to the underlying map.
   */
  final class WriteThroughEntry extends SimpleEntry<K, V>
  {
    private static final long serialVersionUID = -7900634345345313646L;

    WriteThroughEntry( K k, V v )
    {
      super( k, v );
    }

    /**
     * Set our entry's value and write through to the map. The
     * value to return is somewhat arbitrary here. Since a
     * WriteThroughEntry does not necessarily track asynchronous
     * changes, the most recent "previous" value could be
     * different from what we return (or could even have been
     * removed in which case the put will re-establish). We do not
     * and cannot guarantee more.
     */
    public V setValue( V value )
    {
      if( value == null )
      {
        throw new NullPointerException();
      }
      V v = super.setValue( value );
      ConcurrentWeakHashMap.this.put( getKey(), value );
      return v;
    }
  }

  final class EntryIterator extends HashIterator implements Iterator<Entry<K, V>>
  {
    public Entry<K, V> next()
    {
      HashEntry<K, V> e = super.nextEntry();
      return new WriteThroughEntry( e.keyRef.get(), e.value );
    }
  }

  final class KeySet extends AbstractSet<K>
  {
    public Iterator<K> iterator()
    {
      return new KeyIterator();
    }

    public int size()
    {
      return ConcurrentWeakHashMap.this.size();
    }

    public boolean isEmpty()
    {
      return ConcurrentWeakHashMap.this.isEmpty();
    }

    public boolean contains( Object o )
    {
      return ConcurrentWeakHashMap.this.containsKey( o );
    }

    public boolean remove( Object o )
    {
      return ConcurrentWeakHashMap.this.remove( o ) != null;
    }

    public void clear()
    {
      ConcurrentWeakHashMap.this.clear();
    }
  }

  final class Values extends AbstractCollection<V>
  {
    public Iterator<V> iterator()
    {
      return new ValueIterator();
    }

    public int size()
    {
      return ConcurrentWeakHashMap.this.size();
    }

    public boolean isEmpty()
    {
      return ConcurrentWeakHashMap.this.isEmpty();
    }

    public boolean contains( Object o )
    {
      return ConcurrentWeakHashMap.this.containsValue( o );
    }

    public void clear()
    {
      ConcurrentWeakHashMap.this.clear();
    }
  }

  final class EntrySet extends AbstractSet<Entry<K, V>>
  {
    public Iterator<Entry<K, V>> iterator()
    {
      return new EntryIterator();
    }

    public boolean contains( Object o )
    {
      if( !(o instanceof Map.Entry) )
      {
        return false;
      }
      Entry<?, ?> e = (Entry<?, ?>)o;
      V v = ConcurrentWeakHashMap.this.get( e.getKey() );
      return v != null && v.equals( e.getValue() );
    }

    public boolean remove( Object o )
    {
      if( !(o instanceof Map.Entry) )
      {
        return false;
      }
      Entry<?, ?> e = (Entry<?, ?>)o;
      return ConcurrentWeakHashMap.this.remove( e.getKey(), e.getValue() );
    }

    public int size()
    {
      return ConcurrentWeakHashMap.this.size();
    }

    public boolean isEmpty()
    {
      return ConcurrentWeakHashMap.this.isEmpty();
    }

    public void clear()
    {
      ConcurrentWeakHashMap.this.clear();
    }
  }

    /* ---------------- Serialization Support -------------- */

  /**
   * Save the state of the <tt>ConcurrentWeakHashMap</tt> instance to a
   * stream (i.e., serialize it).
   *
   * @param s the stream
   *
   * @serialData the key (Object) and value (Object)
   * for each key-value mapping, followed by a null pair.
   * The key-value mappings are emitted in no particular order.
   */
  private void writeObject( java.io.ObjectOutputStream s ) throws IOException
  {
    s.defaultWriteObject();

    for( int k = 0; k < segments.length; ++k )
    {
      Segment<K, V> seg = segments[k];
      seg.lock();
      try
      {
        HashEntry<K, V>[] tab = seg.table;
        for( int i = 0; i < tab.length; ++i )
        {
          for( HashEntry<K, V> e = tab[i]; e != null; e = e.next )
          {
            K key = e.keyRef.get();
            if( key == null ) // Skip GC'd keys
            {
              continue;
            }

            s.writeObject( key );
            s.writeObject( e.value );
          }
        }
      }
      finally
      {
        seg.unlock();
      }
    }
    s.writeObject( null );
    s.writeObject( null );
  }

  /**
   * Reconstitute the <tt>ConcurrentWeakHashMap</tt> instance from a
   * stream (i.e., deserialize it).
   *
   * @param s the stream
   */
  @SuppressWarnings("unchecked")
  private void readObject( java.io.ObjectInputStream s ) throws IOException, ClassNotFoundException
  {
    s.defaultReadObject();

    // Initialize each segment to be minimally sized, and let grow.
    for( int i = 0; i < segments.length; ++i )
    {
      segments[i].setTable( new HashEntry[1] );
    }

    // Read the keys and values, and put the mappings in the table
    for( ; ; )
    {
      K key = (K)s.readObject();
      V value = (V)s.readObject();
      if( key == null )
      {
        break;
      }
      put( key, value );
    }
  }
}